A side turn signal lamp, which has been known as a prior art, functions as a side turn signal for a diagonal rear of a vehicle only by incident of light onto a light incident end of a light guide lens, and can distribute light with a light quantity which is appropriate to avoid too much brightness for a side of the vehicle. This side turn signal lamp includes: a curved outer lens composed so that a design surface thereof can engage with a laterally extended opening portion of a curved outer sidewall of a door mirror housing; a curved lamp body that is assembled and integrated with a rear of the outer lens, and defines a lamp chamber in cooperation with the outer lens; a synthetic resin-made curved light guide lens that is placed in the lamp chamber, is disposed laterally along the outer lens, and has a light incident end provided on a base end side thereof; and an LED as a light source arranged in a vicinity of the light incident end of the light guide lens. In the side turn signal lamp, the curved light guide lens is composed of a plurality of linear light guides. The linear light guides are substantially circular in cross section and are arrayed up and down so as to approach an inner surface of the outer lens. Each of the linear light guides has reflection steps or stipples on an entire back side thereof. The reflection steps or stipples reflect guided light forward of the light guide concerned (refer to Patent Citation 1).    Patent Citation 1: Japanese Patent Laid-Open Publication No. 2007-250290 (pp. 5 and 6, and FIG. 2)